The packaging of food articles such as fresh red meat, processed meat products, and poultry requires tough, puncture resistant, yet flexible film materials. The films must be substantially impermeable to oxygen, in order to prevent spoilage of the food product and to maintain vacuum where vacuum packaging is employed. It is also important that the films be substantially impermeable to moisture, in order to prevent leakage from within the package.
Oriented amide polymers, hereinafter referred to as nylons, are well known in the packaging industry for their toughness, puncture resistance, and oxygen barrier properties. The oxygen barrier properties of oriented nylon films generally occur when the nylon contains zero or low amounts of absorbed moisture. As the moisture content increases, the oxygen barrier properties of most oriented nylons deteriorate. When the application involves exposure of the film to high humidity or other moist conditions it becomes desirable to encapsulate the nylon between layers having relatively low permeability to moisture, in order to keep the nylon dry. It is also desirable that one of the layers have good heat seal properties. Resins which have both good heat sealability and are substantially impermeable to moisture include various polyethylenes, ethylene copolymers and ionomers. Oriented nylon films are currently used alone and in combination with these heat sealable and moisture resistant layers.
In the packaging of red meat, for instance, an inner layer of polyethylene, ethylene vinyl acetate, or ionomer acts as a sealant layer and prevents permeation of moisture from within the package. A barrier layer of nylon, such as Nylon 6 (polycaproamide) manufactured by Allied Corporation, protects the meat from exposure to oxygen from the atmosphere, thus preventing spoilage. An outer layer of polyethylene or ethylene vinyl acetate protects the nylon from exposure to moisture from the atmosphere.
In a typical process for producing multilayer films containing oriented nylon, the nylon film is oriented by heating to a softened state below the melting point and stretching the softened material. Many conventional nylon resins crystallize very rapidly and have melting points well in excess of the adjacent polyethylene layers. Because the nylon and polyethylene tend to have different stretching characteristics, the nylon must generally be oriented separately and in advance of its combination with the adjacent polyethylene layers. The combination of the oriented nylon with the adjacent layers is then accomplished using a conventional lamination process. This requires a special adhesive, typically a polyurethane type adhesive applied with a coater-laminator.
Another problem with current multilayer oriented nylon structures is that, while the polyethylene generally protects the nylon from moisture, some moisture gradually seeps in from either the packaged food article or the atmosphere and is absorbed by the nylon. This causes an increase in oxygen permeability which shortens the shelf life of sensitive foods.
Because of recent growth in the market for barrier films there currently exists an industrywide search for films with improved barrier properties such as low oxygen permeability and low water permeability. For economic reasons, there is also a demand for an oriented nylon multilayer film which can be produced by a coextrusion process. Production of multilayer films by coextrusion is more economical than lamination methods of the prior art.
It is therefore an object of the present invention to provide a multilayer film structure having low oxygen permeability and preferably low water permeability.
It is a further object of the present invention to provide a multilayer film structure comprising a nylon barrier layer having a low oxygen permeability which does not increase as the nylon gradually absorbs moisture.
A still further object of the invention is to provide a multilayer film structure comprising a nylon barrier layer which can be produced economically by a coextrusion process.
A still yet further object of the present invention is to provide a multilayer film structure comprising a nylon containing barrier layer which may be oriented in combination with polymer materials that have melting points below the glass transition temperature of the nylon.
It is to be understood that additional objectives which will become apparent in view of the specification are also contemplated. Further, these objectives are not to be considered a limitation of the present invention, the spirit and scope of which is delineated in the appended claims.